1. Field of the Invention
The present invention generally relates to an anode assembly and method of reducing sludge formation during electroplating. In particular, the present invention relates to reducing sludge formation during electroplating when utilizing a consumable anode.
2. Description of the Related Art
Reliably producing sub-micron and smaller features is one of the key technologies for the next generation of very large scale integration (VLSI) and ultra large scale integration (ULSI) of semiconductor devices. However, as the fringes of circuit technology are pressed, the shrinking dimensions of interconnects in VLSI and ULSI technology have placed additional demands on the processing capabilities. The multilevel interconnects that lie at the heart of this technology require precise processing of high aspect ratio features, such as vias and other interconnects. Reliable formation of these interconnects is very important to VLSI and ULSI success and to the continued effort to increase circuit density and quality of individual substrates.
As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to sub-micron dimensions, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Many traditional deposition processes have difficulty filling sub-micron structures with relatively severe aspect ratios. Therefore, there is a great amount of ongoing effort being directed at the formation of substantially void-free, sub-micron features having high aspect ratios.
Currently, copper and its alloys have become the metals of choice for sub-micron interconnect technology because copper has a lower resistivity than aluminum, (1.7 xcexcxcexa9-cm compared to 3.1 xcexcxcexa9-cm for aluminum), and a higher current carrying capacity and significantly higher electromigration resistance. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increased device speed. Further, copper has a good thermal conductivity and is available in a highly pure state.
Electroplating is one process being used to fill high aspect ratio features with a conductive material, such as copper, on substrates. Electroplating processes typically require a thin, electrically conductive seed layer to be deposited on the substrate. Electroplating is accomplished by applying an electrical current to the seed layer and exposing the substrate to an electrolyte solution containing metal ions which plate over the seed layer. The seed layer typically comprises a conductive metal, such as copper, and is conventionally deposited on the substrate using physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques. Finally, the electroplated layer may be planarized, for example by chemical mechanical polishing (CMP), to define a conductive interconnect feature.
Typically, electroplating is accomplished by applying a constant electrical current between the anode and the cathode rather than applying a constant electrode potential to the anode or the cathode. In the course of applying a constant electrical current, the voltage of the entire electroplating cell or the potential difference between the anode and the cathode is monitored rather than the potentials at the cathode and at the anode. Due to changes of the processing conditions during electroplating, the electrode potentials of the anode and the cathode vary during the course of electroplating.
One problem with electroplating processes is the formation of particles or sludge in the solution generated as metal is dissolved from a consumable anode, such as a consumable copper anode, during electroplating. The sludge may contaminate or damage the substrates during electroplating. Since cleanliness of the substrates is important for their functionality, contamination by particles should be minimized. Two mechanisms have been proposed for the formation of sludge, such as copper sludge from a consumable copper anode. The first mechanism theorizes that monovalent copper ions (Cu1+) are formed during electroplating in the electrolyte solution which are then both oxidized and reduced to form sludge in the solution. The following reactions illustrate the first mechanism.
xe2x80x832Cu (s) (anode)xe2x86x922Cu1+2exe2x88x92xe2x86x92Cu(s) (in solution as sludge)+Cu2+
The second mechanism theorizes that dissolution of the anode at grain boundaries causes the release of whole metal grains into the electrolyte solution.
One apparatus directed at addressing the problems of sludge formation is the use of a permeable membrane covering the anode. For example, FIG. 1 is a cross sectional view of one embodiment of an anode assembly 10 comprising a consumable anode plate 14, such as a consumable copper anode plate, encapsulated by a permeable membrane 12. The material of the permeable membrane 12 is selected to filter sludge passing from the anode plate 14 into the electrolyte solution, while permitting ions (i.e. copper ions) generated by the anode plate 14 to pass from the anode plate 14 to the cathode. The permeable membrane 12 comprises a hydrophilic porous membrane, such as a modified polyvinylidene fluoride membrane, having porosity between about 60% and 80% and pore sizes between about 0.025 xcexcm and about 1 xcexcm.
One example of a hydrophilic porous membrane is the Durapore Hydrophilic Membrane, available from Millipore Corporation, located in Bedford, Mass. The anode plate 14 is secured and supported by a plurality of electrical contacts or feed-throughs 16 that extend through the bottom of the bowl 18. The electrical contacts or feed-throughs 16 extend through the permeable membrane 12 into the bottom surface of the anode plate 14. The electrolyte solution flows from an electrolyte inlet 19 disposed at the bottom of the bowl 16 and through the permeable membrane 12. As the electrolyte solution flows through the permeable membrane, sludge and particles generated by the dissolving anode are filtered or trapped by the permeable membrane 12. Thus, the permeable membrane 12 improves the purity of the electrolyte during the electroplating process, and defect formations on the substrate during the electroplating process caused by sludge from the anode are reduced. However, one problem with the use of a permeable membrane is that some sludge may still be present outside the permeable membrane. In addition, because of the accumulation of sludge on the permeable membrane, the permeable membrane must be replaced or cleaned.
Another apparatus directed at addressing the problems of sludge formation is the use of a phosphorized copper consumable anode. Typically, a phosphorized copper consumable anode contains about 0.02% to about 0.07% of phosphorous. It is believed that the phosphorous poisons the reaction of the theorized first mechanism of the formation of sludge, discussed above. However, it has been observed that phosphorized copper consumable anodes still produce sludge.
Therefore, there is a need for an improved apparatus and method directed at reducing the formation of sludge.
In one embodiment, a higher applied potential may be provided to a consumable anode to reduce sludge formation during electroplating. For example, a higher applied potential may be provided to a consumable anode by decreasing the exposed surface area of the anode to the electrolyte solution in the electroplating cell. The consumable anode may comprise a single anode or an array of anodes coupled to the positive pole of the power source in which the exposed surface area of the anode is less than an exposed surface area of the cathode to the electrolyte solution. In another example, a higher applied potential may be provided to a consumable anode by increasing the potential of the electroplating cell. A combination of decreasing the exposed surface area of the anode and increasing the potential of the electroplating cell may be used to provide a higher applied potential to a consumable anode.
In another embodiment, an anode may comprise a copper alloy including Ag, Be, Bi, Cb(Nb), Cd, Co, Cr, Fe, Hf, In, Ir, Mo, P, Sb, Se, Sr, Sn, Ta, Te, Th, Ti, Tl, V, Y, Zr, and combinations thereof to reduce the formation of anode sludge.